1. Field of the Invention
The present invention relates to a process and apparatus for removing solids from an aqueous fiber suspension, e.g., a paper fiber suspension, with the aid of gas bubbles. The removed solids are collected in a flotation foam, and discharged as rejects. In this way, the fiber suspension is purified to be constituted by accepted stock suspension.
2. Discussion of Background Information
Processes of the type similar in general that discussed above are used to remove at least a part of the solid particles suspended in a fibrous material-containing suspension. As is known in flotation, foam or scum is formed which contains the materials to be removed. A typical application of this type of process produces an aqueous paper fiber suspension from printed waste paper in which ink particles have already been removed from fibers so that they can be floated out. Thus, this flotation process exploits the differences between fibrous material and undesirable solid particles so that the fibrous material, which is hydrophilic, remains in the fibrous material suspension while the solid particles to be removed, which are hydrophobic, therefore, end up in the foam along with the air bubbles. In addition to ink particles, there are a multitude of other hydrophobic substances that can be removed from the fibrous material via flotation. These substances include, e.g., glues, fine plastic particles, and possibly also resins. If one simply wanted to separate fibers from impurities via flotation, i.e., not all solid particles are to be sorted out, then such a process is referred to as "selective flotation" or "flotation de-inking." As a rule, "flotation de-inking," is not only utilized for the removal of ink particles, but is also generally used for the selective flotation of impurities out of fibrous material suspensions.
The prior art with regard to flotation processes and device for fibrous material suspensions has been extensively developed. Therefore, there are embodiments which are quite suitable for removing a large part of the solid particles via flotation. An important aspect in processes of this type, however, involves which composition and properties are achieved in the flotation foam. Thus, it is desired to remove the flotation foam from the flotation system in the most dewatered form possible, so that removed foam contains the greatest possible quantity of the floated-out impurities and the lowest possible quantity of paper fibers. In general, the flotation foam is collected above the suspension involved in the flotation and is removed, e.g., laterally via a foam weir or vertically upward via a suction device. As a result, dewatering, i.e., drainage, of the flotation foam takes place.